Gougeon Brothers, Inc. regularly engages in testing to support those
who use our epoxy in architectural applications, both in new construction and repairs. In
the past, we have tested the compatibility of house paint primers over WEST SYSTEM® epoxy
and found that a variety of primers worked well. Latex primers especially performed well
when applied over cured epoxy, even when the epoxy was marginally prepared prior to
painting (Epoxyworks 7, Spring 1996).
Our goal in a recent series of tests was to identify primer paints that
worked successfully over WEST SYSTEM epoxy with minimal surface preparation. Specifically,
we wanted to see if primers can be applied satisfactorily to partially cured epoxy.
Applying primer to partially cured epoxy would have the advantage of allowing the
contractor or homeowner to complete the entire repair from epoxy application to topcoat
paint in one day.
The Test
We tested six primers, including ones previously tested in 1996, that
our technical staff have used over epoxy for years on home projects. We also included
fairly new formulations that dry quickly and allow topcoat paints to be applied within an
hour. The majority of primers selected were the fast-dry variety, but one was a
slow-drying oil-base type. The test is fairly simple and something you can do yourself if
your favorite paint is not included in our test.
We started with a sheet of plywood and applied each of our WEST SYSTEM
resin/hardener combinations to separate sections. The epoxy-coated sections were large
enough to allow room for each of the paints to be applied three times. Each epoxy-coated
section was prepared three different ways prior to applying paint. This was done to
identify any paint drying or adhesion problems over certain resin/hardener combinations.
Group 1 involved epoxy-coated samples where the epoxy had cured
hard and had been prepared with our normally recommended surface prep of washing with
water and sanding dull with 100 grit sandpaper prior to applying paint.
Group 2 involved epoxy that had cured hard and had been sanded dull
with 100 grit sandpaper without washing with water prior to applying the primer paints.
Group 3 involved applying primer paints while the epoxy was
partially cured, without any additional surface preparation. The paints were applied when
the epoxy achieved a slightly tacky to soft "rubber like" cure. You may recall
that this approach was first discussed in Epoxyworks 14, Fall 1999–Porch Railing
Coating Analysis.
By varying surface preparation on each of our resin/hardener
combinations, we hoped to identify primer paints that worked well over a variety of epoxy
cure and surface prep conditions. If there are paint types that have a problem curing over
epoxy or if paints cannot adhere well to epoxy under certain conditions, we wanted to
know.
Our shop was 80°F and 70% relative humidity when the epoxy was
applied. The high humidity was ideal for this test because if amine blush is going to form
on epoxy, it will most likely occur when humidity is present. If blush has an effect on
paint drying, we would see it in our results.
The Primers
Bulls Eye 1-2-3 Primer Sealer (water-based) from Wm. Zinsser Co. is
an interior/exterior primer that dries in 30 to 45 minutes and allows recoating with latex
or oil-based topcoat paints after one hour. It is able to adhere to a variety of surfaces
with minimal surface preparation and offers bleed through resistance to bleeding type
stains.
Fresh Start™ from Benjamin Moore™ is 100% Acrylic Latex
(water-based) primer that dries quickly and allows you to topcoat with finish paint (any
oil or latex paint in Benjamin Moore's line) within three hours of application. It is able
to adhere to a variety of surfaces with minimal surface preparation and offers bleed
through resistance to bleeding type stains.
MOORE'S™ Latex Exterior Primer from Benjamin Moore & Co.
is a fast-dry, water-based primer. It dries in one hour and allows recoating with topcoat
latex paint in three hours.
Cover-Stain Primer Sealer from Wm. Zinsser Co. is a white pigmented
interior/exterior oil-based paint that dries in 30 to 45 minutes and allows recoating with
oil or latex topcoat paints in one hour. It seals bleeding wood stains and can be applied
to shiny surfaces without surface prep.
XIM Primer is a quick-dry, solvent-based sealer/bonder that dries
in 10 to 20 minutes to the touch and is ready for topcoating with oil or latex paint in
one to two hours.
MOORWHITE™ Primer is an exterior grade alkyd primer that dries
in 6 to 8 hours and can be recoated with topcoat paint after overnight drying. It is
unique in that it remains a bit flexible and can be recoated with either oil or latex
paints. Since this paint had a difficult time drying over epoxy in our 1996 tests, we
wanted to retest it.
Results
The table shows that all of the primers performed well when 1) the
epoxy was washed and sanded or 2) when the cured epoxy was just sanded. Most also show
encouraging results when 3) applied over partially cured epoxy.
Adhesion testing of the paint films was done 24 hours after the paint
was applied. A car key was dragged across the surface and the adhesion was deemed
acceptable if the paint stayed attached to the epoxy.
No adhesion problems were found when paint was applied to cured epoxy.
When the paint was applied over partially cured epoxy, two of the paints, Bulls Eye 1-2-3
and MOORWHITE oil-base primer, were a bit soft after 24 hours and could be scraped off.
However, this was a short-term adhesion problem. After a few days of additional drying,
adhesion improved to acceptable levels. To put this in perspective, I can say that in the
past few years I have personally used Bulls Eye 1-2-3 primer successfully on home projects
over partially cured epoxy without any adhesion problems.
Mud cracking
Another oddity that occurred to paints applied directly to partially
cured (and still sticky) epoxy was "mud cracking". We believe this paint crazing
phenomenon occurs because paint films shrink as they dry. The paint doesn't crack during
normal use because it is usually applied to a solid. In this case, though, the paint dried
on a semi-liquid (the partially cured epoxy) and mud cracks formed because the paint
wasn't physically attached to a solid substrate.
We have seen mud cracking occur on projects at home and simply waited
for the paint to dry and applied another coat of primer over the cracked sections. Because
the epoxy was cured further, the paint did not mud crack again and adhesion was fine.
Recommendations
Many primers can work well over WEST SYSTEM epoxy using a variety of
surface preparation and cure situations. When possible, it is best to allow the epoxy to
cure. However, primers are available that allow a one day turnaround of repairs to painted
wooden structures with WEST SYSTEM products. Armed with this knowledge, you can feel
confident about using these primer products over WEST SYSTEM epoxy.